The present invention generally relates to High Aspect Ratio Contact trench etching and, in particular, relates to the reduction of charge buildup along the trench sidewalls during High Aspect Ratio Contact trench etching.
Successful construction of nano- and microstructures requires reliable and reproducible methods of production. One such nano- or microstructure is a contact hole, or trench. Contact hole structures are generally fabricated using wet (crystal anisotrophy) or dry plasma (ion-bombardment anisotrophy) etching. One example of a contact hole formed by dry plasma etching is shaped by etching through an oxide layer overlaying a silicon substrate using a hard photoresist mask deposited on top of the oxide layer, wherein the etching substantially stops on the underlying substrate layer. Contact holes have a diameter, also known as width, and a depth. The diameter is referred to as the feature size and tends to decrease with increasing circuit density. The aspect ratio is the ratio of depth to width and tends to increase as the width decreases. Modem integrated circuits are scaled with increasingly narrower design rules. In addition, as the width of the etched features decreases, the aspect ratio increases, necessitating a high aspect ratio contact trench etch process.
Therefore, high aspect ratio contact (HARC) trench etching is one of the key processes for forming contact hole interconnections. In typical plasma etching, positive ions are accelerated to the substrate by a radio frequency (RF) biased electrode sheath providing directionality for forming vertical contact hole profiles. The substrate layer is disposed on a chuck and placed within the gas chamber. The chuck acts as a bottom electrode and can be biased by a second RF power source. During plasma etching, plasma electrons, due to their random thermal motion, tend to impinge on the sidewalls near the top of the contact hole causing charge accumulation. Charge accumulation is one of the main causes of charge build-up damage, etching stop, as well as micro-loading effects.
Carbon chain polymers are a result of the plasma etching. Conductivity of the sidewalls in the contact holes increases during the etching processes resulting in carbon chain polymer buildup along the sidewalls of the contact hole. These deposited carbon chain polymers strongly affect the sidewall conductivity in the contact holes. The source of the carbon that form the carbon chain polymers may be from the hard photoresist mask, from the carbon source plasma etching gases, or from the oxide layer itself. Over the course of the etch process, the bottom of the contact hole charges positively while the sidewalls charge negatively, thereby creating undesired local electric fields within the contact hole.
During typical HARC etches, this charge buildup along the sidewalls of a narrow and deep opening can deflect the incoming ions causing changes in the trajectory of those ions. This, in turn, results in the contact hole twisting during its formation and becoming non-vertical. Further, sidewall charging may also lead to complete etch stoppage in HARC contact holes. Another related issue associated with the charge buildup along the sidewalls is that the contact hole misses the active area landing region in the underlying substrate due to the twisting of the contact hole during its formation. Therefore, it is important to produce vertically straight contact holes because straight sidewall profiles ensure that the subsequently deposited metal material can properly fill the etched feature and make suitable electrical contact with the active area landing region.
Therefore, there is a need for a method to reduce charge buildup along the carbon chain polymer which forms along the sidewalls of the contact holes during HARC etching in order to produce substantially vertical contact holes.
There is also a need for a method to produce substantially vertical contact holes without shutting off the etch component of the HARC etching.
In addition, there is a need for a method which increases the step coverage of the carbon chain polymer buildup along the sidewall in order to enable the charge buildup to bleed off.